The present invention relates generally to transducer arrays, and more particularly, to spiral transducer arrays manufactured using micromachining fabrication technologies.
Capacitive micromachined ultrasonic transducers (CMUTs) in particular have been fabricated in this manner. Spiral sparse arrays have been described in various publications, Spiral sparse arrays are discussed in U.S. Pat. No. 5,808,962 entitled xe2x80x9cUltrasparse, Ultrawideband Arraysxe2x80x9d and a technical paper by Sumanaweera et al. entitled xe2x80x9cA Spiral 2D Phased Array for 3D Imagingxe2x80x9d published in the Proceedings of the IEEE International Ultrasonic Symposium, 1999.
Capacitive micromachined ultrasonic transducers (CMUTs) have also been described in various publications. Such transducers are described in U.S. Pat. No. 5,619,476 entitled xe2x80x9cElectrostatic Ultrasonic Transducerxe2x80x9d, U.S. Pat. No. 4,262,339 entitled xe2x80x9cFerroelectric Digital Devicexe2x80x9d, and U.S. Pat. No. 4,432,007 entitled xe2x80x9cUltrasonic Transducer Fabricated as an Integral Part of a Monolithic Integrated Circuitxe2x80x9d.
Finally, the following papers report the use of micromachining technologies in the fabrication of conventional ultrasound transducer designs: (1) R. A Noble et al., xe2x80x9cNovel silicon nitride micromachined wide-bandwidth ultrasonic transducersxe2x80x9d, presented at the 1998 IEEE International Ultrasonics Symposium in Sendai, Japan, (2) X. C. Jin, xe2x80x9cMicromachined capacitive transducer arrays for medical ultrasound imagingxe2x80x9d, presented at the 1998 IEEE International Ultrasonics Symposium in Sendai, Japan, (3) I. Ladabaum, xe2x80x9cMiniature drumheads: microfabricated ultrasonic transducersxe2x80x9d, Ultrasonics 36 (1998) 25-29, and (4) H. T. Soh, xe2x80x9cSilicon micromachined ultrasonic immersion transducersxe2x80x9d, Appl. Phys. Lett. 69 (24), Dec. 9, 1996.
However, heretofore, the use of micromachining has not been applied to the fabrication of spiral arrays and sparse spiral arrays in particular. Spiral arrays, previously recognized as offering unique beam-forming advantages such as sidelobe elimination, have not been rendered manufacturable using conventional transducer construction methods. Inventors of the present invention recognize the unique abilities of micromachining are now able to solve this problem in advantageous manners disclosed herein.
In the past, conventional two-dimensional arrays (areal arrangements of piezoelements) have been fabricated using piezoelectric ceramic materials such as PZT. Although the typical ceramic PZT materials used in medical ultrasound transducer arrays have a high dielectric constant, the electrical impedance of a small two-dimensional array element is very high. This prevents effective transmission of the transmission pulse signals through the transducer cable without using buffer amplifiers at the probe end of the cable.
In addition, the electrical connection to the small areal piezoelectric ceramic elements is generally done using multilayer flexible circuits, which comprise a layered structure of polymer and metal support materials, typically Kapton(trademark) and copper. Kapton, having a low acoustic impedance, and copper having a high acoustic impedance, form a highly undesirable acoustic loading to the high acoustic impedance piezomaterial. This in effect increases the internal undesired reflections within the transducer and compromises the necessary temporal compactness of the transducer""s acoustic output in order to get good axial resolution.
It would be desirable to have a transducer structure wherein electrical connections do not significantly compromise the acoustic signal quality. It would also be desirable to have a transducer structure manufactured using micromachining fabrication techniques and materials that overcome the limitations of conventional arrays. It would also be desirable to have improved ultrasound imaging systems employing such transducer structures.
The present invention provides for spiral, or substantially spiral, transducer arrays manufactured using micromachining techniques and materials, with the arrays preferably being capacitive micromachined ultrasonic transducer arrays. Capacitive micromachined ultrasonic transducers (CMUTS) have been demonstrated to have sensitivities that are equivalent to piezoelectric ceramic elements.
Before proceeding the terms xe2x80x9cmicromachiningxe2x80x9d and xe2x80x9cmultilayer interconnectsxe2x80x9d used herein shall be defined.
Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining which we utilize-1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface-and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.
Multilayer interconnects are defined as including interconnects made in the manner of IC or integrated circuit interconnects or interconnects found on hybrid circuit substrates. More particularly, the transducer embodiments disclosed herein incorporate at least two of the following known items or two instances of one of the known items:
(1) Thin-film interconnect layer such as those deposited by PVD, CVD, LPCVD, electroplating, electroless plating, screen-printing, pattern-forming dispensing techniques or damascene-type CMP or chemical-mechanical-polishing techniques,
(2) Diffused interconnect layer or ion-implanted interconnects,
(3) Silicide metal-based interconnect layer,
(4) Vias or contact through-hole layer as formed by wet-etching, dry plasma etching, laser drilling, chemical photodevelopment of a photosensitive polymer dielectric or screen-printing,
(5) Interlayer insulating dielectric layer such as thin-film PECVD glasses, SOGs and spin-on polyimide, and
(6) Overcoat layer such as hermetic oxynitride or nitride protective and insulating layers used in combination with one or more of (1-5).
It is to be emphasized that the interconnects and vias may be either (or both) surface features (limited to the surface films as in a typical IC) or bulk features such as through-the-wafer vias and interconnects found in micromachined silicon pressure sensors sold by the millions.
By eliminating conventional fabrication processes and using micromachined processes and materials, the inventors of the present invention even more importantly realized that the difficult interconnection routing problem inherent to spiral arrays can be solved in addition to getting rid of the above impedance mismatch problems. Micromachining technologies are utilized for the fabrication of acoustic elements and related IC multilayer interconnection technologies to also solve the interconnection routing problem among those elements and their supporting electronics. Specific arrangements of such multilayer interconnects are described in support of micromachined spiral arrays and sparse spiral arrays.
The batch fabrication techniques and constructions described herein eliminate the exceedingly difficult and expensive challenge of trying to use unsuitable conventional technologies to bring spiral arrays to product fruition.
An exemplary spiral sparse array comprises a silicon substrate or wafer further comprising a spiral array, or substantially spiral arrays, of capacitive micromachined ultrasonic transducer elements (CMUTs). The capacitive micromachined ultrasonic transducer elements may specifically be disposed in the shape of an exponential spiral, for example. The capacitive micromachined ultrasonic transducer elements (vibratable membranes typically) may be inexpensively batch-manufactured using the well-established silicon micromachining manufacturing technologies whose typical steps are outlined in the above items (A)-(C) widely known to the art; for example in the current micromachined accelerometer markets and pressure-sensor markets. Batch fabrication of micromachined arrays will all for inexpensive disposable transducers.
Further, multilayer interconnection technologies described in items (1)-(6) above are utilized to enable solving of the spiral array interconnect problem by incorporating sufficient interconnect layers to allow interconnect routing within the areal outline of the array itself. Such interconnection technologies are widely known in the IC art and hybrid circuit art.
Specifically, preferred arrangements utilizing at least two interconnect layers and at least one contact (via) layer serving the spiral elements and their associated circuitry are envisioned. The two or more layers may be entirely in the surface films of the array (i.e., surface micromachining and interconnection) or may include through-substrate vias or interconnects (i.e., bulk micromachined devices such as pressure sensors and accelerometers).
A preferred embodiment of the invention is the combination of multilayer interconnect and micromachining as applied to solving the spiral array manufacturability issue.
A plurality of amplifiers are preferably individually coupled to each transducer element of the spiral array. The electronics of the imaging system is coupled to each of the amplifiers and thereby allows generation of acoustic output (or acoustic reception as desired) from the substantially spiral sparse array. The plurality of amplifiers overcomes the electrical impedance mismatch between the CMUT transducer-membrane elements and the electronics of the imager. The use of multilevel interconnection technologies allows the amplifiers (or other per-element circuitry) to be cointegrated in or on the same substrate as the array elements themselves.
An additional embodiment mates the amplifiers (or other per-element electronic circuits) by using ball-grid array interconnects (BGAs) to connect an array chip to a juxtaposed and aligned circuitry chip. This embodiment allows the array and its electronics to be separately yieldable as subcomponents.
Another embodiment also utilizes a separately made array chip and circuitry chip, but instead of face-to-face BGA interconnects, the interconnection is done generally laterally using thin or thick film interconnects in the manner of known multichip modules or multichip hybrids.
In the case of a substrate comprising a silicon wafer or a silicon-coated wafer (or other semiconductor material), the supporting per-element circuitry for the array may comprise integrated circuitry formed in said silicon in the conventional manner. The array acoustic elements may be formed using micromachining processes practiced on the substrate before, during or after the IC formation processes as is widely known in the art. In any design, the circuitry, if incorporated on the same chip as the acoustic elements, is located such that it does not block the acoustic propagation path. (e.g., circuitry under the elements or beside the elements, for example).
It is important to note that although CMUTs are the preferred elements, one may also utilize other micromembrane-based micromachined elements. Such alternatives include piezo-film coated micromembrane PMUT) whose vibration is excited (or sensed) instead by the electrically-driven piezofilm coating on the membranes.